In chemistry, metallurgy, environment and other industry, complex coupled processes including multiphase flow, mass transfer, heat transfer and reaction are often involved in which the flow is the basis for all the other processes, and will largely determine the efficiency of other processes. Thus, velocity measurement of multiphase flow is very important for industrial reactor design, optimization and amplification.
Although the fluid velocity measurement technologies have been greatly developed, there is still a huge challenge in multiphase flow velocity. At the early stage, pitot tube and thermal anemometer are mainly uses. For pitot tube, the dynamic response frequency is so low that the information of high-frequency pulse in unsteady turbulent field cannot be recorded. High viscosity liquid or fine solid particles will clog pitot probe. So its application has certain limitations. Thermal anemometer is divided into hot wire anemometer and hot film anemometer. Hot wire anemometer has high frequency response, high sensitivity, large dynamic range and small size of the probe. But the probe and sensor are easily broken in a strong vortex containing particles. In addition, high viscosity liquid is easier to stick on the probe surface which affects measurement accuracy. Hot film anemometer is relatively solid and wear-resisting, but its frequency response is low.
The development of laser technology promotes the rapid development of the laser doppler velocimetry (LDV) and particle image velocimetry (NV). LDV, because of its high precision and fast response, has been widely used in gas-solid, gas-liquid, solid-liquid flow experiments. But the LDV has strict requirements for the concentration of measured solid phase particles or fluid tracer. At the same time, complex instrument calibration needs to be done before it is used. PIV can get the instant information of the whole flow field in unsteady flow. But due to the complexity of multiphase flow of tracer particles, PIV technique is usually used only to measure the liquid phase velocity in multiphase flow. Particle tracking velocimetry (PTV) is a special kind of PIV, which can calculate the velocity by tracking the trajectory of individual particle and achieve high spatial precision. But the current PTV technique has great demands on measured flow channel, tracer or dispersed particles and image processing.
Therefore, the measuring method of the particle velocity in multiphase reactors has yet to be further studied.